I am running a channel flow using LES. My problem is that turbulence after developing for a bit starts to decay in channel. And after a while I see zero turbulence in my domain. What can I do under such circumstances to sustain the turbulence. Thank you for your answer.

Accurate LES modelling is not easy, I hope you are not a CFD beginner. If you have not heard of the -5/3 line or never drawn a turbulence spectrum then I suggest you do some study of turbulence theory before proceeding.

A major problem in LES modelling is dissipative advection schemes causing energy to lost numerically before it trickles down the subgrid model. To counter this most LES models use central differencing as it does not have the numerical dissipation of the default upwinding schemes.

CFX is using central difference scheme, yet it cant sustain the turbulence. I know how to draw energy spectrum and I understand the basic physics of LES. I will be thankful if you can give any hints/suggestions to model plane channel flow so that I can sustain turbulence and get fully developed flow.

I am using periodic boundaries and I have set pressure drop between inlet and outlet. Do I need to set up pressure gradient as well under subdomain???

You are using translational periodic boundaries with a pressure drop? That should be sufficient, the flow rate should then adjust so the pressure drop in the domain equals the pressure rise is the periodicity.